# Can someone sanity check my basic electronics please?

Hi guys,

Been looking at this and the maths works out but it doesn’t ‘feel’ right for some reason and I can’t work out why…

Looking at using the USBizi100 to control a specific transistor.

The transistors are: [url]http://uk.farnell.com/nxp/bc817-40/transistor-npn-45v-0-5a-sot23/dp/1081225[/url]

I am trying to calculate the resistor to limit the base current going from the pin to the BC817-40. The USBizi pin is at 3.3V with a max current of 4mA per pin.

Current check: The min hfe of the transistor is 250 with a fully on Ic of 500mA so this should be 2mA from each pin to fully turn it on. I need 32 transistors connected so max package current needs to be kept to a minimum so this would be 64mA so should be fine.

Voltage check: The USBizi runs at 3.3V. The transistor Vbe is 1.2V so this should be fine as the datasheet for the LPC2387 says a minimum of 2.5V when high.

So according to this Rb = (3.3-1.2)/0.002 = 1050Ohm so pick next nearest below = 1k

Would this work? As I say it just doesn’t feel right…

Thanks in advance and my apologies for being a bit dumb on this one.

Andy

What you did is ok, instead you are using the wrong numbers.

Worst case VBE is 1.2V max. In case VBE was lower than 1.2V your USBbizi current would be higher than 2mA…

Why do you use 500mA as Ic what are you going to switch with this transitor ?
Why not use a low power MOS instead ?

That might be why it didn’t feel right. Thanks Pablo, will have a hunt for a MOS.

//A

Thor,

Take into account that you need a MOS that could fire with 2.5V which is the minimun of the uC.

Pablo,

Thanks for that. This one should work shouldn’t it:

It has a threshold voltage Vgs 2V typ.

//A

Sorry for the late input.

Note that that mos has a Vgs(th) of 2V, but that means that it STARTS conducting at 2V. Study Figure 7. At Vgs of 3V you can expect a drain current of max ±70mA. Their stated drain current of 475mA is at a Vgs of 10V.

I have burnt my fingers on this before…

It is the same package, but rated for 6A with a Vgs of 10V and, according to Figure 3, it can do that from a Vgs of 1.75V.

Thanks,
Errol

Hi Errol,

Thanks for the input, it is very much appreciated.

If I went with this MOS I shouldn’t need a limiting resistor between the USBizi and gate pin should I? (Assuming this as there is only 700pF of Input Capacitance)